You will be clear about this issue after this...
First...There is no such animal as 'air refuelable drop tank'. If I guess correctly, you wonder if it is possible to refuel
ONLY the external fuel tank. Technically speaking, there are no engineering obstacles for that. But it is not wise to do that. It is dangerous because you will have to refuel the starboard wing external, then move to refuel the port wing external, then what about the centerline external ? You must also consider asymmetric mass/weight as each external wing tank is being filled.
Second...Here is how fuel management really works...
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Internal fuel means fuselage fuel tanks (plural). Of course, these tanks differs in shape, quantity, and locations from jet to jet. Next are wings internal fuel tanks. Last are external fuel tanks, which consists of wings and centerline.
Let us keep things simple for now.
The feed tank means exactly that -- it feeds the engine.
1- As the feed tank is being used, fuel from other fuselage tanks supplies the feed tank.
2- As the fuselage tanks are being used, the wings internal tanks supplies the fuselage tanks.
3- As the wing internal tanks are being used, the external tanks supplies the wings internal tanks.
The precise tank number that supplies the next tank number and in what sequence is unique to each aircraft regarding weight distribution/balance, and that get much more technically complex. You pretty much have to go back to the manufacturer for the details, assuming you have the necessary clearance and 'need-to-know'. But what I am explaining here is public information.
Refueling is the reverse. The fuselage tanks are the first to fill, then they supplies the wings internal, then the externals, if there are any.
The entire process is called 'cascade fuel' management, meaning flowing from one tank to the next, consumption or refuel. It does not matter if it is ground or air refuel. The flow is still the same. The jet may take fuel from the ground receptacle or the in-flight refuel receptacle (IFR) -- the cascade flow is still the same.
The only time you can refuel the external tanks is ground refuel, but not air. Basically, you just open up the fuel cap and stick the hose in. But if you want to refuel the entire jet, then hook up an external power supply to the jet and refuel the entire jet through the main ground refuel receptacle and let the jet's fuel management do the rest. If there are any external wing and centerline tanks, they will be refueled and refueled last.
Hope that helped.
